…ch logic

This change reverts the coordinator side to be much simpler, as it was
before CL 167382, and consolidates all the version selection in maintapi.
This direction is chosen because at this time¹ maintapi is best suited
to perform version selection.

This is a refactor CL that leaves current behavior unmodified, and test
cases provide improved coverage. The following smaller CL will make the
desired changes to behavior.

Remove TestNewTrySetBuildRepoGo110 because it's no longer needed. It was
added when x/build started requiring module mode to build successfully,
given that Go 1.10 didn't have module mode support.

Background

At this time¹ coordinator makes use of maintapi to find TryBot work,
and it is a collaborate effort between the two components that results
in determining what builds will happen. Coordinator is ultimately
responsible for starting and running the builds, but it doesn't have
information about the branches in the Go project (the Go revision at
refs/heads/master, refs/heads/release-branch.go1.16, etc.). Maintapi
has that information via the maintner corpus. So it makes the version
information available to coordinator by populating relevant fields in
apipb.GoFindTryWorkResponse.

Issue golang/go#28891 was about wanting to test golang.org/x repo CLs
on release-branch.go1.n with the corresponding Go 1.n version, rather
than Go tip. Unfortunately, it was implemented on the coordinator side,
resulting in the logic for version selection to be more spread between
the coordinator and maintapi components. There were followup issues
like golang/go#42127 and golang/go#37512, whose fixes built on top of
the coordinator side, and increased complexity there. As a consequence,
making and testing further changes became more difficult than it needs
to be.

¹ After golang/go#34744 is done, I'd like to move all the TryBot version
  selection logic into coordinator, the component responsible for TryBots.
  But not today.

For golang/go#46154.

Change-Id: I93986acefd4bf66b27ccf0323439966122b7989a
Reviewed-on: https://go-review.googlesource.com/c/build/+/319789
Trust: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>